In recent years, with the shrinking supply of more valuable light hydrocarbon feedstocks, it has become increasingly important to employ heavy hydrocarbon feedstocks in the production of petrochemicals. This is especially the case due to the demand for light hydrocarbons, i.e. gaseous olefins such as ethylene, propylene, butadiene etc., monocyclic aromatics such as benzene, toluene and xylene etc. and naptha. Accordingly, methods for the production of these lighter petrochemicals form heavy feedstocks have been developed in the art.
However, in all of these processes, the thermal cracking of the heavy hydrocarbons results in significant amounts of coking which leads to a stoppage in production due to fouling of the process equipment. Further, in catalytic cracking, the heavy hydrocarbons often contain a large amount of metals which poison the catalyst, thus requiring expensive catalyst regeneration or replacement of the catalyst.
Recently, the production of lighter hydrocarbons has been reported with some success in a process which employs the addition of a transition metal catalyst complex and very fine particulates to the heavy hydrocarbon feedstock. See, U.S. Pat. Nos. 4,770,764 and 4,863,887. These processes have proved to be relatively insensitive to feed metals. See, FIG. 1, which shows in graphic form the percentage of demetalation as a function of conversion by these processes.
However, in these processes, as the conversion level is increased to above about 60%, a marked increase in coking is observed. See, FIG. 2, which shows, in graphic form, the percentage of coke yield as a function of percent conversion by these processes. Thus, there remains in the art a need for a process which can operate at high conversion without significant coke formation, yet have a reduced need for catalyst replacement due to poisoning.
To this end, the present Applicants have surprisingly found a novel process combination which satisfies these long felt needs in the art.